Why do my renders look Flat?
It would be helpful to see your node setup, to know which rendering engine you're using (Eevee, Cycles, or Blender Render if you're using 2.79 or earlier) and which version of Blender you are using.

However, I attempted to reproduce your issue using an approximately similar shape and this is what I found:

A rough approximation of your flat rendered shape.

A jury-rigged version of your shading here.

Node setup for this shading.


If this is what your node setup looks like, this is incorrect. You should add a Shader node first, connect it to the Surface input of the Material Output node, and add your to that Shader node, as shown below.


Result of above node setup.


I hope that this helps, but more details will help illuminate the problem!